9. Taal, Philippines (2.38 million/24.8 million):Taal is a lake-filled caldera that produced four VEI 4 eruptions in the last 200 years and a VEI 6 eruption only ~5,500 years ago (VEI stands for Volcanic Explosivity Index, and it tops out at 7). Combine that explosivity with abundant water to add to potential explosive eruptions and the large population that could be impacted by ash, and you have a very closely-watched volcano. Taal is monitored by PHIVOLCS, the Philippine volcano monitoring agency.

8. Coatepeque Caldera, El Salvador (1.2 million/6.5 million):Coatepeque is the first "dark horse" in the top 10. It gains points for erupting rhyolite and dacite, both magmas prone to large, explosive eruptions. It is also centrally-located in El Salvador, so a large eruption would likely impact the capital of San Salvador along with the city of Santa Ana. Like Taal, it is a lake-filled caldera, adding to its potential danger by potentially increasing explosivity or mudflows (lahars).

7. Corbetti Caldera, Ethiopia (1.2 million/9.8 million): Now, this is a real under-the-radar volcano. The Corbetti caldera lies within an even older caldera and has produced pyroclastic cones (explosive eruptions of lots of volcanic debris) and obsidian flows, meaning it has the right style of eruption and right composition to potentially experience a big explosive eruption. Not much is known about the Corbetti Caldera, so it is hard to constrain its recent activity. However, it is close enough to Addis Ababa that a large ash-rich eruption might cause quite a humanitarian crisis.

6. Tatun Group, Taiwan (6.7 million/9.8 million): Much like the Corbetti Caldera, Tatun is not a well-known volcano in a country most people don't associate with volcanism. However, as I wrote about recently, the Tatun Group has all the signs of a volcano that is still potentially active. It is also nestled close to Taipei, so you could imagine an eruption that produced another andesite dome could wreak havoc on the city, mainly from ash fall or mudflows.

5. Vesuvius, Italy (3.9 million/6.0 million): Did you really think Vesuvius wouldn't be in the top five? The volcano is one of the most dangerous on Earth thanks to its numerous explosive eruptions—and the city of Naples, which is slowly crawling up its flanks. The fact that it doesn't fall at the top of this list (heck, it's not even the most dangerous in Italy) betrays how hazardous the other volcanoes might be. Vesuvius has been quiet since 1944, so we're full into the "complacency" phase where most people don't remember the last eruption—never a good place to be when 6 million people could be impacted by an explosive eruption.

4. Ilopango, El Salvador (2.9 million/6.7 million): This is another caldera in El Salvador. But unlike Coatepeque, it has erupted in the last 200 years (1880 to be exact). Around 450 CE, Ilopango had a VEI 6 eruption that covered much of El Salvador with ash and brought down Mayan cities across the region. Today, San Salvador sits directly next to the this lake-filled caldera, so the significant danger from this caldera remains after 1,500 years.

3. Aira Caldera, Japan (0.9 million/2.6 million): The population around the Aira caldera might be lower than most of the top 10 volcanoes, but its frequent eruptions (from Sakurajima) and history of large eruptions means it poses a large danger to those 2.6 million people within 100 kilometers. Over the past 10,000 years of the Holocene, the Aira caldera has had a half dozen VEI 4, 5, and 6 eruptions—so don't be fooled by the constant din of smaller explosions from Sakurajima over the last decade.

2. Michoacan-Guanajuato, Mexico (5.8 million/5.8 million): Here's the thing about the Michoacan-Guanajuato (M-G) volcanic field: All three population radius values are the same: 5.8 million. Yes, almost 6 million people live within 5 kilometers of this volcanic field that has produced pyroclastic cones generated by explosive eruptions. It has produced numerous VEI 3 and 4 eruptions over the Holocene from 1,400 vents. This means it hasn't had big eruptions like some of the top 10. But the frequency, potential explosivity, and population in the widespread volcanic area makes it a high risk.

1. Campi Flegrei, Italy (3.0 million/6.0 million): If you're the sort of person who wants to worry about Yellowstone, maybe you should turn your attention to the Campi Flegrei instead. Not only is it a restless caldera with a more recent history of very large explosive eruptions, it is also smack in the middle of an area with over 6 million people ... and it's partially under the Bay of Naples. All these factors mean that if the Campi Flegrei has a new bout of explosive eruptions, the hazards could exceed those of any eruption in modern history. That being said, the last eruption (Monte Nuovo in 1538) was actually a fairly-benign cinder cone.

Now, this ranking is highly subjective. There can be a multitude of ways to measure danger, so I'm sure people will disagree with this list. Volcanoes like Etna, Cotopaxi, Ruiz, Fuego, and more didn't make the top 20—mostly because I chose to put emphasis on the style and composition of magmatism.

The big important point here: This list highlights the most potentially "dangerous" volcanoes based on their past behavior (mainly) and the potentially for mass casualties. There will always be volcanoes that might only have a few thousand people living near it that could erupt and kills hundreds of people.

I think of eruptions like El Chichón in 1982 as a great example. The volcano wasn't even really recognized as a threat until it erupted and killed around 1,900 people. It is unlikely that we can eliminate all volcanic threat, and as the global population grows, the danger posed by volcanoes we can identify as hazardous—and those we don't recognize as hazardous—will only increase. Funding volcanic research and monitoring along with emergency management organizations is the only way we can hope to protect ourselves from major volcanic disasters.